The present invention is directed to aircraft engine turbine engines with thrust augmentors, and more specifically to augmentors or afterburners with closed trapped vortex cavities.
High performance military aircraft typically include a turbofan gas turbine engine with thrust augmentors or afterburners to provide additional thrust when desired, such as when transitioning to supersonic flight. The turbofan engine includes in downstream serial flow communication, a multistage fan, a multistage compressor, a combustor, a high pressure turbine powering the compressor and a low pressure turbine powering the fan. A bypass duct surrounds and allows a portion of the fan air to bypass the multistage compressor, combustor, high pressure turbine and low pressure turbine.
During operation, air is compressed in turn through the fan and compressor and is mixed with fuel in the combustor. The fuel is ignited in the combustor, generating hot, high energy combustion gases which flow downstream through the turbine stages. The turbine stages extract energy from these combustion gases. Hot core gases exiting the turbine stages are then discharged into an exhaust section of the engine, which includes augmentor or afterburner hardware. The gases traversing the exhaust section are discharged from the engine through a variable area exhaust nozzle, the gases providing thrust which drives the aircraft.
Afterburners are located in the exhaust sections of the turbine engines. The exhaust sections include an exhaust casing and an exhaust liner circumscribing a combustion zone. Fuel injectors (such as spraybars) and flame holders are mounted between the last stage of the turbine section and the exhaust nozzle. These injectors add additional fuel into the exhaust nozzle which when ignited, provides augmented thrust that accelerates the aircraft. Thrust augmentation or reheat using such fuel injection is referred to as wet operation, while dry operation refers to unaugmented flight when the afterburners or augmentors are not actively operational.
An annular bypass duct extends from the fan to at least the augmentor, bypassing a portion of the fan air around the core engine to the afterburner. The bypass air is mixed with the core gases and fuel from the augmentor spray bars, ignited, and combusted prior to discharge through the exhaust nozzle. The bypass air is also used in part for cooling various engine components, such as, for example, the exhaust liner.
Various types of flameholders are known and provide local low velocity recirculation and stagnation regions therebehind, in regions of otherwise high velocity core gases, for sustaining and stabilizing combustion during reheat operation. Since the core gases are the product of combustion in the core engine, they are initially hot, and are further heated when burned with the bypass air and additional fuel during reheat operation. Augmentors currently are used to temporarily maximize thrust and tend to be full stream, that is consuming all available oxygen in the combustion process, yielding high augmentation ratios, for example, a ratio of about 70%.
In regions immediately downstream of the flameholder, the gas flow partially recirculates and the velocity of the gas flow is less than the rate of flame propagation. In these regions, there will be a stable flame existing that can ignite new fuel as it passes. Unfortunately, flameholders in the gas stream inherently cause flow losses and reduced engine efficiency. Several modern gas turbine engines and designs include radially extending spraybars and flameholders in an effort to improve flame stability and reduce the flow losses. Radial spraybars integrated with radial flameholders are disclosed in U.S. Pat. Nos. 5,396,763 and 5,813,221. Radial spraybars disposed between radial flameholders having integrated radial spraybars have been incorporated into the GE F414 and GE F110-132 aircraft gas turbine engines. This arrangement provides additional dispersion of the fuel for more efficient combustion and unload fueling of the radial flameholders with the integrated radial spraybars so that they do not blow out and/or have unstable combustion due to excess fueling.
Since fuel is typically injected upstream of the flameholders, undesirable auto-ignition of the fuel and combustion which might occur upstream of the flameholders causes flameholder distress, which also significantly reduces the life of the flameholders. Since V-gutter flameholders are suspended within the core gases, they are more difficult to effectively cool, and typically, experience circumferential variation in temperature, which correspondingly affects thermal stress, decreasing the useful life thereof. V-gutter flameholders have limited flameholding capability and their aerodynamic performance and characteristics negatively impact the size performance and thrust capability of the engine. This is, in part, due to the combustion zone having sufficient length to allow substantially complete combustion of the fuel added by the spraybars prior to discharge through the nozzle with wide ranging flight speeds and Mach numbers.
Recent advances in flameholder design such as the trapped vortex cavity pilot disclosed in U.S. Pat. No. 8,011,188 issued Sep. 6, 2011, assigned to the assignee of the present invention and incorporated herein by reference in its entirely have been developed. In this design, ignition of fuel for the augmentor is provided by an ignition system formed in an annulus of the of the combustion liner. The annulus is a counterbore forming a cavity in the combustion liner extending 360° around the exhaust, having a forward wall and an aft wall. The cavity also includes an outer cavity wall positioned radially outward of the combustor line. As the cavity is a counterbore in the combustion liner. there is no wall forming a boundary with the exhaust gas flow. As used herein, the terms forward and aft are used to describe a position of a feature with respect to the engine, forward referring to features positioned more toward the front of the engine, aft refers to features positioned toward the engine exhaust, while radial positioning is described with reference to the engine centerline. The ignition system includes cooling holes and fuel injector tubes positioned to inject air and fuel into the cavity forming an air/fuel mixture. This air/fuel mixture is ignited by igniters operably positioned around the cavity. The specific design of the cooling holes determines the shape of a vortex within the cavity forming the pilot flame when ignited by the igniters. This pilot flame is then used to ignite air and fuel in the exhaust when the engine undergoes wet operation, that is, when augmentation is required. The open trapped vortex created within the cavity is in direct communication with the core flow.
Despite the above-described existing technology, there remains a need for an augmentor with a flame stabilization apparatus that has better performance characteristics than previous flame stabilization apparatus used to ignite afterburners or augmentors.